Dames & Moore, 1999 - USDA Forest Service

Crystalline crusts were observed in the mill building and where seepage emerges along the toes of the
tailings piles. The USGS used X-Ray Diffraction (XRD) to determine the types of minerals present in
1994 (personal communication with Jim Kilbum, 1999) (Table 6.1-1). ~ lminerals l were identified to be
sulfates in combination with aluminum, iron, copper, potassium, sodium, magnesium, zinc, and calcium.
These minerals are not unique to the Holden Mine and have been documented at many other mine sites.
This section summarizes the hydrology/hydrogeology discussed in Sections 4.3 and 4.4, and describes a
general conceptual site transport pathway model for the Site. The model serves as the basis for
interpretation of surface water and groundwater PCOC discharge from Site features as they relate to water
quality in Railroad Creek.
6.13.1 General Transport Pathway Model
The Site receives surface water and groundwater flow from upgradient sources that include snowmelt and
rainfall runoff from adjacent valley sides and from direct precipitation as either rain or snow.
Groundwater within the Railroad Creek valley and at the Site exists as shallow and deeper isolated
occurrences. Shallow groundwater occurs near the base of the tailings piles and within a relatively thin
near-surface alluviurn/reworked glacial till (reworked sand and gravel) that is perched above less
permeable glacial till. Site groundwater within these units discharges as seeps or as diffuse groundwater
to Railroad Creek. The alluvium/reworked till unit is considered to be moderately permeable with
estimated hydraulic conductivities ranging from 9 x lo4 to 5.3 x cdsec. The underlying glacial till
unit is a relatively low permeability material that is assumed to separate the groundwater that occurs in
the bedrock fractures from the groundwater that occurs in the overlying alluviudreworked till. The
underlying glacial till unit may contain limited localized occurrences of water. The glacial till unit is
estimated to have lower permeabilities than the alluviudreworked till unit, with hydraulic conductivities
ranging from lo-* to 1 0-lo CIIL~S~C.
The interrelationship between Site groundwater and surface water is the focus of the transport pathway
discussion and includes the following mechanisms: (1) surface water overland flow and infiltration, (2)
seepage as overland flow, (3) seepage as direct groundwater discharge to Railroad Creek, and (4)
groundwater discharge as baseflow into the bed of Railroad Creek. Other transport pathways such as air
transport of tailings or transport of tailings directly into Railroad Creek as a result of bank erosion are
considered to be a minor influence on water quality in Railroad Creek.
In general, upslope surface water from direct precipitation and snow melt run-on infiltrates and recharges
groundwater through fractures within the bedrock found along the valley sidewalls and through the pore
spaces of the surficial deposits including the alluviudreworked till unit, where present. Upslope surface
water run-on results in overland'flow and infiltration across and through Site features and eventually
discharges to Railroad Creek as overland flow or as diffuse groundwater. Baseflow to Railroad Creek is
provided by. diffuse groundwater flow which likely flows in alluvium underlying the creek.
Seeps discharges from the surficial materials at the Site at points where local groundwater elevations are
higher than ground elevations. Seepage may (1) discharge directly into Railroad Creek, (2) flow overland
to Railroad Creek, and (3) flow overland and reinfiltrate. In the spring, most water enters the Site from
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DAMES & MOORE
17693-005-019Wuly 27,1999,4:11 P~RAFI' FINAL RI REPORT